1. Field of the Invention
The present invention relates to a method of preparing a crystalline ingot of Hg.sub.1-x.sbsb.o Cd.sub.x.sbsb.o Te of a substantially uniform composition by the THM method (Traveling Heater Method) in which:
at the bottom of a substantially cylindrical vertical enclosure is disposed a solidified mixture of Te, Hg Te and Cd Te in proportions corresponding to those of the liquid phase, at said predetermined temperature (T) of said ingot to be prepared, PA1 a cylindrical source ingot of Hg Te and a cylindrical source ingot of Cd Te are disposed in contact with said solidified mixture, the ratio of their respective sections being equal to the desired Hg/Cd ratio of said prepared ingot, PA1 said enclosure is closed, under a vacuum, PA1 the zone of the enclosure corresponding to said solidified mixture is heated so as to melt this latter and to bring it to said predetermined temperature (T), thus obtaining a solvent zone for dissolving the parts of said source ingots in contact with the solvent zone, and PA1 said heated zone is moved along said enclosure so as to move said solvent zone and to progressively dissolve said source ingots at one of the ends thereof and so as to obtain, at its other end, the crystallization of said Hg.sub.1-x.sbsb.o Cd.sub.x.sbsb.o Te ingot to be prepared. PA1 before disposing said solidified mixture at the bottom of said enclosure, a monocrystalline germ of Hg.sub.1-x.sbsb.1 Cd.sub.x.sbsb.1 Te is disposed at the bottom of said enclosure such that: EQU x.sub.o &lt;x.sub.1 .ltoreq.1 PA1 said germ being provided with a flat and polished face. PA1 and after disposing said solidified mixture in the bottom of said enclosure, in contact with said solidified mixture is disposed a piston of a density less than that of the solvent having a flat base, said enclosure is closed, under a vacuum, without disposing said source ingots inside, the zone of the enclosure is heated corresponding to said solidified mixture so as to bring the solvent obtained to said predetermined temperature, for the time necessary for dissolving the part of said germ which has been work hardened by polishing, then the mixture is cooled at a given rate so as to obtain the crystallization of an adaptation zone, said enclosure is opened and said piston is withdrawn so as to dispose said source ingots in contact with said solidified mixture.
The crystalline ingots thus prepared are used for manufacturing infrared radiation detectors or else avalanche diodes. Such devices are obtained by known processes of doping a substrate cut from such an ingot.
2. Description of the Prior Art
A method of the above defined type is already known, described in French Pat. No. 81 05387 in the name of the applicant. This method allows crystalline ingots to be prepared having the shape of a circular cylinder of a diameter of the order of 20 mm. By cutting such a cylinder into slices of appropriate thickness, a large number of disks is obtained. Each of these disks forms a flat wafer from which, considering its relatively large area, a large number of electronic components of the above type may be obtained, whose area is relatively small. The disk is then cut up so that each component is available individually. However, a relatively large amount of these components, having characteristics out of tolerance, must be rejected. This is due to the fact that these components, in order to operate satisfactorily, must be formed on a monocrystalline substrate, that is to say formed of a crystal having, on the macroscopic scale, an orientation and only one, and only having defects if there are any, on the atomic scale. Now, the Hg.sub.1-x.sbsb.o Cd.sub.x.sbsb.o Te ingot obtained by the above mentioned method is of a polycrystalline structure, that is to say formed, on the macroscopic scale, of a plurality of monocrystals of different orientations, of maximum dimensions of the order of 10 mm, separated by grain joints and having crystalline defects. The electronic components to be rejected are those which are formed on the parts of the disk comprising the joints and the defects. To avoid such rejects, the consequence of which is harmful in so far as the manufacturing costs are concerned, it would then be necessary to use monocrystalline disks of defined orientation.
A method of manufacturing such a monocrystalline layer of Hg.sub.1-x.sbsb.o Cd.sub.x.sbsb.o Te is known by epitaxy in the liquid phase from a monocrystalline substrate of Cd Te, for example. However, the thickness of such a layer is limited to 10 microns or so, and the method, which only allows one layer to be manufactured at a time and requires a new germ each time, is costly.
In order to obtain using the THM method a monocrystalline ingot of appreciable volume, attempts have already been made to use a monocrystalline germ of Cd Te, for example, previously disposed at the bottom of the preparation enclosure. However, it has been discovered in this case that, at the beginning of the process for drawing the ingot, a beginning of the dissolution of the germ by the solvent occurs which disturbs the appearance of the composition balance which normally takes place within the solvent zone. This uncontrolled phenomenon introduces longitudinal variations of the composition of the ingot obtained, in which there still exists a considerable density of crystalline defects, and sometimes grain joints. A zine or selenium doping reduces this crystalline defect density, but the grain joints still exist.